Conversion of an analog signal to digitally coded form results in a finite number of quantitizing steps, even if the characteristic of the utilized analog/digital (A/D) converter is ideal. The errors arise in the region of the least significant bits (LSBs). The quantitizing errors are due to the finite number of quantitizing steps of the signal. A discrete value of an analog signal may be represented in digital form, after quantitizing and coding into the digital form by a predetermined digital value represented by a data word of a predetermined value; alternatively, it may be represented by an immediately adjacent data word. The analog signal value, however, actually, may be between both the first data word value as well as the immediately adjacent data word value. It is, thus, a matter of chance or random occurrence whether, in temporally sequential digitizing circuits, the respective analog value is represented by the first data word, or the next adjacent data word. In case of low analog signal levels, the quantitizing ambiguity of .+-.1/2 LSB becomes noticeable based on a percentage of data words being considered. If the analog signal only has low-frequency components, the quantitizing ambiguity becomes cumulative.
Reduction of noise in a reconverted analog signal which had previously been digitized is described for example in German Patent Disclosure Document DE-OS No. 31 04 247. In the there described process, the quantitizing error which occurs upon digitizing an analog signal is intended to be reduced. It is first determined when an if a digital signal includes only low-frequency information; if the answer to the interrogation is "yes", adjacently positioned interrogation values of the signal are averaged by a filter in order to reduce the noise level or noise content. The system has a disadvantage, namely of reducing the resolution of the analog signal since two or more adjacent interrogation values are averaged. This reduction in resolution is undesirable. Conversion of analog signals to digital form and subsequent re-conversion is frequently desirable, since signals can be transmitted over a transmission system easier in digital than in analog form.